Documents & Sync High Data Use Despite Turning Off iCloud Sync & Using Low Data Mode

iPhone 8 Plus, iOS 13.2.3

I first uncovered this issue two weeks ago when I was notified that I'd used 90% of my data for the month. Self-diagnosed iCloud sync issues as Documents & Sync had used up 5 GB of data in ~1 week. Here is what I did initially to troubleshoot the problem:

1. Logged out, then back into iCloud, while on Wifi
2. Turned on Low Data Mode
3. Toggled iCloud Drive to "Off" under the Cellular menu
4. Turned Photos to "Off" under the Cellular menu
5. Turned off "Wifi Assist"
6. Cleared my "Cellular Data" settings so I could more accurately track data usage

Since Sunday, 11/24, I've used 583 MB of data. Roughly half of that was through normal usage- Google Maps, ESPN Fantasy Football, Amazon, Twitter, Chrome. 280 MB was used by System Services with 234 MB of that for Documents & Sync. More specifically, between yesterday morning and this morning System Services has used 220 MB of data, almost entirely for Documents & Sync.

I am trying to figure out what is still causing the high data usage in Documents & Sync despite turning off the usual high users of cell data. During this period the only time I was not connected to Wifi was the 15 minute drive to my in-law's house for Thanksgiving or the 15 minute drive back to my home afterwards. Outside of that, the only other time I was not connected to Wifi was during a ~1 hour walk I took last night. I tracked the walk using the Workout app on my watch (Series 3, NO cellular) and I listened to a podcast I had already downloaded to the phone using Overcast. I believe workout tracking would have shown up under "Time & Location" and probably "Mapping," not "Documents & Sync." If the podcast had streamed for some reason, I would think it would have shown up under Overcast.

Outside of that, I'm kind of at a loss as to what could have caused this amount of data usage in a 24 hour period when I'm actively trying to slow data usage and spent most of that time on WiFi. Any ideas/suggestions? Thanks!